Project Summary The STAT transcription factors play pivotal roles in controlling the expression of genes involved in immune responses, cell transformation and maintaining homeostasis. Published results from this lab demonstrate that there is a pool of STAT3 that is localized in the mitochondria (mitoSTAT3) where it functions to control cellular respiration in cells and permit Ras transformation of mouse embryo fibroblasts. Phosphorylation at serine 727 of mitoStat3 regulates the production of reactive oxygen species (ROS), activity of complex I of the electron transport chain, and tumorigenesis of breast cancer cells. In addition to the actions of mitoSTAT3 serine 727 on tumorigenesis, we have identified a novel signaling cascade stimulated by hydrogen peroxide, IL-6 and oncostatin M treatment of cells which results in a rapid loss and re-accumulation of STAT3 in the mitochondria. RE-entry of mitoSTAT3 coincides with binding of mitoSTAT3 to cyclophilin D (CypD). CypD regulates the mitochondrial permeability transition pore (MPTP) that opens in response to ROS and other stressors, leading to mitochondrial swelling and cell death. We hypothesize that the interactions of CypD with mitoSTAT3 modulate opening of the MPTP to regulate cell proliferation and apoptosis. This proposal will examine how these signaling pathways function to control cellular ROS and control growth of normal and transformed cells.